Accurately determining the location of mobile embedded devices (such as smartphones) in indoor environments can be difficult. A global positioning systems (GPS) cannot be used because the needed satellite signals are hard to receive indoors because they are blocked by the walls of the building.
There are a number of existing approaches to indoor localization. One approach is a WiFi®-based indoor localization (WiFi® is a registered trademark of the WiFi Alliance in Austin, Texas). In general, this type of approach records the signal strength from WiFi® access points in the immediate vicinity. Given the location of the WiFi® access points, the location of the mobile device can be calculated. Similar approaches have also been used where FM radio transmitters deployed in the building are used instead of WiFi® signals.
Another approach is proximity-based indoor localization. This type of approach uses a large number of low-power radios (such as RFIDs, low-power Bluetooth® devices (Bluetooth® is a registered trademark of the Bluetooth® Special Interest Group in Kirkland, Wash.), FM radio transmitters) that are deployed in every room or location that needs to be localized. The mobile embedded device detects proximity to and obtains its location from the nearest low-power radio source or sources.
Each of these approaches measure a signal strength between the mobile embedded device and the transmitter. The indoor space of the building then is profiled by creating a map of the signal strength. For example, this profiling may occur by measuring the signal strength along every meter of the indoor space and recording each of the record wireless access points that can be connected to at each location. This collection (or vector) of signal strengths becomes the “fingerprint” of that indoor location. This is performed for a large numbers of locations within the building to create a fingerprint database that consists of pairs of ground truth locations and signal strength vectors.
Once the fingerprint database is obtained, any user can enter the building with his mobile embedded device and localize the device. The mobile embedded device will determine which local radio transmitters are within the range of the device, and will record the signal strength from these individual transmitters at the user's current location in the building. The fingerprint of the mobile embedded device at the location in the building is compared to the fingerprint database to find the closest match. The position that is associated to the closest fingerprint match in the database is assumed to be the location of the mobile embedded device in the building.
One problem, however, with using local radio transmitters (such as WiFi®) is that it operates at the 2.4 GHz range, which means that its signal strength is susceptible to human presence, device orientation, and presence of small objects in a room. Additionally, these parameters change over time, further impacting the signal strength of WiFi® signals, and leading to additional errors in the indoor localization.